LARGE SCIENTIFIC PROJECT
Gravitational-Wave Astronomy
Figure 6. Gravitational-wave sources detected by the LIGO-Virgo-KAGRA( GWTC4.0 catalog) Source: https:// www. ligo. caltech. edu / news / ligo20250826
measurement of the speed of gravitational waves: their velocity matches that of light to within one part in 10 15, ruling out a wide class of modified gravity theories that had been proposed to explain dark energy. Thanks to the localization capabilities of the Virgo-LIGO network, about 70 telescopes around the globe, spanning almost the entire electromagnetic spectrum, were able to quickly point toward the source, located in the galaxy NGC 4993, about 130 million light-years away. Among them was the Hubble Space Telescope. This coordinated effort led to the discovery of an optical transient, the socalled kilonova, produced by nuclear reactions following the merger of the two neutron stars. The spectral and photometric analysis of the kilonova light confirmed the presence of heavy elements( such as gold, platinum, and lanthanides), thus validating the long-standing hypothesis that such mergers are key sites for r-process nucleosynthesis [ 15 ]. Moreover, this joint detection enabled a new method to measure the Hubble constant, by combining the gravitational-wave luminosity distance with the electromagnetic redshift of the host galaxy. This so called standard-siren approach provides a completely independent way from those using Type Ia supernovae or the cosmic microwave background, and is therefore essential in addressing the ongoing Hubble tension [ 16 ].
Technological advancements and the rise of compact objects population studies
Around 2018, one of the most significant upgrades was introduced into the detectors: the implementation of squeezed vacuum states of light. This technique, originally theorized in the early 1980s by Carlton Caves, reduces quantum noise, particularly at high frequencies, by manipulating vacuum fluctuations. The application of squeezed light marked the beginning of a new generation of quantum-enhanced gravitational-wave detectors [ 17 ]. In 2019, data-taking resumed for the third observing run( O3). Between 2019 and early 2020, before the interruption caused by the COVID pandemic, nearly 100 gravitational-wave sources were detected. This volume of events allowed the community not only to highlight exceptional signals, but also to begin conducting population studies. By combining the properties of multiple sources, researchers could derive statistical distributions of mass and spin for observed black holes and begin investigating whether these distributions are consistent with theoretical predictions. For example,
Figure 5. Virgo, source: https:// www. ligo. caltech. edu / image / ligo 20170927b
the third gravitational-wave transient catalog( GWTC-3), published in 2021, revealed an excess of black-hole with masses near 35 solar masses, raising questions about their mechanism of formation [ 18 ]. These studies opened the door to a statistical astronomy of compact objects, no longer focused solely on rare or spectacular events, but on the global structure and demographics of the gravitational-wave sky.
Toward a global network: new detectors, KAGRA and the O4 data taking
After 2020, the detectors underwent further changes. In particular, LIGO managed to increase its power and implemented a specific form of squeezing, called frequency-dependent squeezing. Virgo introduced the socalled signal recycling mirror( already present in LIGO), with the aim of enhancing the detector’ s bandwidth, and the optimization of this configuration is still underway. Meanwhile, in 2023, data collection resumed with a fourth observing run( O4) involving the international network of terrestrial interferometric detectors, currently known as LVK( LIGO – Virgo – KAGRA), and soon to be called IGWN,
52 www. photoniques. com I Photoniques 134